1. Field of the Invention
The present invention relates to a metal oxide resistor; a power resistor suitable as a fixed resistor, a variable resistor, and a resistor array, which are used in a high-voltage unit or a charge/discharge unit of a bulk capacitor or the like; and a power circuit breaker having the power resistor as a closing resistor.
2. Description of the Related Art
Generally, the resistor materials are roughly classified into metallic resistor materials, metal oxide resistor materials, and non-metallic resistor materials. Of these materials, the metal oxide resistor materials have heat resistances and energy breakdowns for absorbing a high electrical energy, which are higher than those of other materials.
Typical metal oxide resistors are disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 58-139401 and 59-217668.
Jpn. Pat. Appln. KOKAI Publication No. 58-139401 describes a carbon grain dispersion ceramic resistor having a sintered body obtained by dispersing a conductive carbon powder in an insulating aluminum oxide crystal and sintering them by a clay. This sintered body of the resistor contains 1.5 to 2.0 wt % of the carbon powder.
Jpn. Pat. Appln. KOKAI Publication No. 59-217668 describes another carbon grain dispersion ceramic resistor having a sintered body. This sintered body is obtained as follows. A carbon powder and a binder are added to an insulating inorganic material consisting of aluminum oxide, mullite, and a calcined clay. They are mixed, kneaded, and heat-treated to coat the particles of the insulating inorganic material with carbon. The carbon-coated particles are sintered as a starting material, thereby obtaining the sintered body. The sintered body of the resistor contains 1.5 to 5 wt % of a fine carbon powder having a size of 0.1 .mu.m or less.
Generally, when a carbon powder is added to an aluminum oxide powder, the sintering properties of the aluminum oxide are degraded. In the carbon grain dispersion ceramic resistors described in the above patent applications, since the carbon powder is added to the aluminum oxide powder, the sintering properties of the aluminum oxide are impaired. For this reason, in the above-described carbon grain dispersion ceramic resistors, after the carbon powder is added to the aluminum oxide powder, the clay is added to compensate for the sintering properties of the aluminum oxide. However, the added clay only serves to bind the aluminum oxide powder and the carbon powder. For example, a sintered body sintered by a clay has a sectional structure, as schematically shown in FIG. 1. In this sintered body, as shown in FIG. 1, a large amount of a clay 2 and carbon particles 3 exist throughout the grain boundaries of metal oxide particles 1, and voids 4 are also formed therein. Since the resistor has low denseness, i.e., a porosity of 10 to 30%, the following problems are posed.
That is, since the sintered body of the resistor has a high porosity, when the resistor is exposed at a high temperature, the carbon particles for controlling the resistance are oxidized. This causes large variations in resistance to make the resistor unusable. In addition, the sintered body of the carbon grain dispersion ceramic resistor has the nonuniformly dispersed carbon powder, as shown in FIG. 1. Therefore, it is difficult to obtain a desired resistance and a satisfactory strength.
When the resistor is used as a closing resistor of a power circuit breaker connected in parallel with a breaking connection point to absorb a surge generated during a switching operation or increase the breaking capacity, the temperature of the resistor rises significantly in accordance with heat generation caused by surge absorption because the sintered body has a high porosity, i.e., poor denseness, and its heat capacity per unit volume is as small as about 2 J/cm.sup.3.deg. With an increase in temperature of the resistor, the carbon is oxidized as described above, or a defect is caused due to a thermal expansion between the sintered body and electrodes or inside the sintered body. As a result, when the carbon grain dispersion ceramic resistor is to be used as a closing resistor, a resistor with a large volume must be used to ensure the necessary heat capacity per unit volume. Therefore, a large space is required to store the resistor, resulting in a bulky circuit breaker.